Truncated bimodal latitudinal diversity gradient in early Paleozoic phytoplankton

Abstract

The latitudinal diversity gradient (LDG)—the decline in species richness from the equator to the poles—is classically considered as the most pervasive macroecological pattern on Earth, but the timing of its establishment, its ubiquity in the geological past, and explanatory mechanisms remain uncertain. By combining empirical and modeling approaches, we show that the first representatives of marine phytoplankton exhibited an LDG from the beginning of the Cambrian, when most major phyla appeared. However, this LDG showed a single peak of diversity centered on the Southern Hemisphere, in contrast to the equatorial peak classically observed for most modern taxa. We find that this LDG most likely corresponds to a truncated bimodal gradient, which probably results from an uneven sediment preservation, smaller sampling effort, and/or lower initial diversity in the Northern Hemisphere. Variation of the documented LDG through time resulted primarily from fluctuations in annual sea-surface temperature and long-term climate changes.

Supplemental Material: Latitudinal diversity gradient dynamics during Carboniferous to Triassic icehouse and greenhouse climates

“…S2-S4; see a description in the Supplemental Material). Similar patterns have recently been reported for other icehouse and greenhouse climate-transition periods (i.e., a unimodal LDG appearing before global cooling at the Eocene-Oligocene boundary [Crame, 2020] and multiple late Ordovician peaks [Zacaï et al, 2021]). However, the Cenozoic greenhouse and icehouse climatic alternation seems to show a consistent picture of tropical diversity decline and midlatitude diversity (Kiessling et al, 2012;Boag et al, 2021).…”

Cooling Oceans Did Trigger Ordovician Biodiversification

“…Here we use a coupled paleoclimatic and macroecological model 20,21 to evaluate the individual contributions of extrinsic mechanisms, namely the long-term Ordovician cooling trend 8-10 and paleogeographical evolution 11,22 , to the trajectory of marine biodiversity during the Ordovician (between 490 to 430 Ma). In contrast with previous attempts based on temporal correlations, our approach allows us to quantify processes and establish causal relationships.…”

Palaeoecology of Cambrian–Ordovician acritarchs from China: evidence for a progressive invasion of the marine habitats

“…Today, most acritarchs are widely considered to be the cysts of marine phytoplankton (e.g. [19,33]). As such, they must have played an important role in the marine trophic chain.…”

“…the organic-walled phytoplankton, were focused on analyses of their diversity (e.g. [5,18,19]). Nowak et al [20] revised the global acritarch diversities during the Cambrian, completed by Zheng et al [21], who provided additional Chinese data.…”

“…Recently, Zacaï et al [19] added a palaeogeographical scenario to the global diversity patterns, by providing data concerning the latitudinal diversity gradient of Cambrian–Ordovician acritarchs. These authors identified a southward drift of peak diversity during the Cambrian, a colonization of intertropical latitudes during the Early Ordovician, and a subsequent duplication in the diversity peak during the Middle Ordovician.…”